Seismic expansion joint covers for buildings in geographic regions that are prone to earthquakes are commonly of special designs that allow for movements of the building elements on either side of the expansion gap that are very much greater than the movements that occur as a result of thermal expansion and contraction. Most seismic expansion joint covers follow traditional design philosophies that have long been applied to expansion joint covers that are not intended to sustain earthquakes; they use metal covers and various fastening systems to join the covers to frame members that are attached to the building members on either side of the expansion gap in such a way as to retain the covers in place in the gap during seismic events while permitting the large motions of the members.
In a commonly used fastening system, the cover is attached by bolts to the centers of spaced-apart bridge bars that span the gap with their ends sliding in trackways in the frame members. As the gap expands and contracts, the bridge bars pivot about the connecting bolts. An example of a seismic expansion joint cover system that uses bridge bars is found in Moulton U.S. Pat. No. 5,078,529 issued Jan. 7, 1992, for "Seismic Expansion Joint Cover."
Previously known seismic expansion joint covers can sustain relatively weak earthquakes but are severely damaged, often beyond repair, in severe earthquakes, such as the one that occurred in Jan. 1994, just north of Los Angeles. An inspection of several installations of seismic expansion joint covers of various designs in the Los Angeles area just after that earthquake revealed bent and mangled covers, failed connectors, frames ripped from their anchors in the walls and floors, and damage to the walls adjacent the covers caused by impacts of the partially detached covers against the walls. Few of the inspected installations were repairable.